The present invention relates to a magnetron used for a device utilizing a microwave such as a microwave oven.
The aforesaid magnetron is configured in a manner, as an example, that a magnet ring (also called as a conversion plate or a shim plate) formed by magnetic material is disposed between an anode cylinder having anode vanes disposed radially on the inner wall surface thereof and an circular magnet disposed on the opening end side of the anode cylinder, in order to improve a magnetic force in an active space at the periphery of a cathode structure (refer JP-A-2002-163993, for example). Since the magnet ring is disposed, much magnetic flux can be conducted into the active space, so that the efficiency of a magnetic circuit can be improved.
In order to improve the efficiency of the magnetic circuit, it is important to conduct much magnetic flux in the active space at the periphery of the cathode structure. However, since the magnet ring of the magnetron of the related art has a uniform thickness, the magnetic flux is also attracted on the outer periphery side which does not so much contribute to the conduction of the magnetic flux in the active space at the periphery of the cathode structure, so that there is a limit in the improvement of the efficiency of the magnetic circuit.
Further, since the magnet ring of the magnetron of the related art has an annular uniform shape, the magnet ring can not be restricted and so free in the radial direction. Thus, the positioning of the magnet ring is difficult in the manufacturing process of the magnetron. As a result, the magnet ring likely deviates at its center axis from the radial direction of the circular magnet and the anode cylinder. When the center axis of the magnet ring deviates, the magnetic characteristics in the active space at the periphery of the cathode structure can not be realized so as to coincide with the design. Thus, there arises a problem that the oscillation becomes unstable and desired oscillation efficiency can not be achieved. In order to solve this problem the magnetron disclosed in JP-A-2002-163993 is configured in a manner that, as shown in a perspective view of FIG. 14 and a partial sectional diagram of FIG. 15, a cut and erected part 100a is provided at the outer circumferential part of a magnet ring 100, and then the cut and erected part 100a is fit into the outer circumferential part on the cathode structure side of an circular magnet 110 to thereby position the cut and erected part to thereby prevent the positional deviation in the radial direction of the magnet ring 100 and the circular magnet 110. The JP-A-2002-163993 also discloses another example having a cut and erected part provided at the inner circumferential part and a still another example having an arbitrary number of projection portions provided on the entire surface on the side contacting to the circular magnet.
However, the magnetron disclosed in the JP-A-2002-163993 provides the cut and erected part at the outer circumferential part or the inner circumferential part of the magnet ring or provides the arbitrary number of projection portions on the entire surface in order to suppress the positional deviation of the circular magnet and the magnet ring. There arises a problem that the cut and erected part or the projection portions obstructs the flow of the magnetic flux lines to thereby degrade the efficiency of the magnetic circuit.